Bell inequality violation with an atom-compatible polarization-entangled NIR–telecom biphoton source from a warm atomic ensemble

Quantum networks are rapidly evolving toward hybrid architectures that interconnect distinct physical platforms. Here, we report a practical atom-compatible source of bright polarization-entangled biphotons (1529 nm for signal photon and 780 nm for idler photon) generated from a warm atomic ensemble via spontaneous four-wave mixing (SFWM). By systematically investigating the spectral properties of the SFWM process at 780 nm, we identify frequency shifts of the generated biphotons and efficiently suppress scattering noise from strong pump fields using etalon filters. The intrinsic selection rules inherent to the SFWM process enable direct generation of polarization-entangled photon pairs, in particular the maximally entangled Bell state |ψ⁺⟩. We observe strong polarization correlations and obtain a CHSH S-parameter of 2.75 ± 0.03 for net coincidences, corresponding to a strong violation of the Bell inequality. Furthermore, we experimentally verify near-resonant atom–photon interaction using a 780 nm idler photon whose center frequency is ~2.5 GHz blue-detuned from the 5S₁⁄₂ (F = 2)–5P₃⁄₂ (F′ = 3) transition of ⁸⁷Rb, highlighting the suitability of our source for atomic-vapor–based quantum network architectures.